Aluminum electrical conductor and process for making the same

ABSTRACT

THIS DISCLOSURE RELATES TO ALLOYS OF ALUMINUM SUITABLE FOR USE AS ELECTRICAL CONDUCTORS. THE ALLOYS CONTAIN FROM IN EXCESS OF 1% TO ABOUT 3% BY WEIGHT IRON AND ARE TREATED SO THAT THE PARTICLE SIZE AND DISTRIBUTATION OF THE INTERMEDIATE PHASE CONTAINING IRON ARE CONTROLLED WHEREBY   THESE IRON CONSTITUENTS PARTICLES IMPART STRENGTH, DUCTILITY AND RESISTANCE TO SOFTENING AT ELEVATED TEMPERATURES TO THE ALUMINUM WITHOUT PROHIBITIVELY DEGRADING ITS CONDUCTIVITY.

P. P. ZEIGLER ETAL mg. 6, 1974 v ALUMINUM ELECTRICAL CONDUCTOR AND PROCESS FOR MAKING THE SAME Filed May 12, 1971 =3 imfivdmm mmmaqwql w w M w w a w m m w m n w m n a n n 0 w Q 5 5 E w a Q M a L Y m2 G 0% M3 N5 mi m u u n u u w W W M mu Q vfiiiubmsu EFFECT OF HEAT/N6 ON THE CU/VDUCT/V/TY AND HARD/V555 0F COLD WORKED Al- 2% Fe ALLOY R00 PAUL R ZE/GLEE $5 A TTOE/VEY United States Patent 3,827,917 ALUMINUM ELECTRICAL CONDUCTOR AND PROCESS FOR MAKING THE SAME Paul P. Zeigler, Oakland, and Sidney G. Roberts, Livermore, Calif., assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif.

Continuation-impart of abandoned application Ser. No. 834,333, June 18, 1969. This application May 12, 1971, Ser. No. 142,505

Int. Cl. (122i? N04 US. Cl. 148-2 13 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to alloys of aluminum suitable for use as electrical conductors. The alloys contain from in excess of 1% to about 3% by weight iron and are treated so that the particle size and distribution of the intermediate phases containing iron are controlled whereby these iron constituent particles impart strength, ductility and resistance to softening at elevated temperatures to the aluminum without prohibitively degrading its conductivity.

CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part application of copending US. application Ser. No. 834,333 filed June 18, 1969, and entitled, Process for Making Aluminum Electrical Conductor Alloys, now abandoned.

BACKGROUND OF THE INVENTION Aluminum has come into increasing use as an electrical conductor. Ductility is important in electrical conductor alloys so that twisted connections can be made without breaking the conductor. Pure aluminum is an excellent conductor having an electrical conductivity of about 65% IACS (65 of that of the International Annealed Copper Standard). However, to have sufiicient strength for most conductor applications, it must be severely cold worked. The amount of strengthening that can be achieved .by cold working is limited by the work-hardening capacity of the material and the loss in ductility that accompanies severe deformation. At the present time, relatively pure aluminum (99.45% Al minimum) designated as BC or electrical conductor grade aluminum, is often cold worked to a tensile strength of about 27,000 p.s.i. At this strength, the material has a tensile elongation of about 1 /2 to 2% in inches.

Aluminum may also be strengthened by the addition of alloying elements or by combined alloying and cold working. The most effective alloying elements currently used to strengthen aluminum have appreciable solubility in aluminum and only small quantities of these can be used without causing intolerable losses in electrical conductivity. Thus, aluminum alloys used for electrical conductors contain only very small quantities of magnesium or magnesium plus silicon.

The effect of iron and other addition elements in small amounts on the conductivity of aluminum is reported by G. G. Gauthier in The Conductivity of Super-Purity Aluminum: The Influence of Small Metallic Additions, J. of the Inst. of Metals, vol. 59 (1936), pp. 129-146. In this work, small amounts of iron ranging from 0.22% to 0.99% were added to high purity aluminum (purity exceeding 99.99%) and the conductivities measured. Gauthier found that iron in these amounts had little effect on electrical conductivities. No consideration was given in regard to strength and ductility properties.

In 1948, R. H. Harrington published an article entitled, The Effect of Single Addition Metals on the Recrystallization, Electrical Conductivity and Rupture Strength of Pure Aluminum in the Transactions of the American Society for Metals. This article noted that the addition of 1% iron, which was usually regarded as an undesirable impurity, resulted in an alloy possessing outstanding rupture strength at 257 F. in the annealed condition while maintaining a high electrical conductivity of 61% in good agreement with conductivities previously reported by Gauthier. Nothing significant seems to have been done with this early recognition that iron could be added to the material without significant loss of electrical conductivity, probably because the material made by Harrington did not show a significant improvement in ductility over normal electrical conductor grade aluminum.

SUMMARY OF THE INVENTION It has been found that a desirable combination of strength, conductivity, and ductility in an electrical conductor material may be achieved at a tolerable economic cost by processing aluminum containing iron in amount from in excess of 1% to about 3% by weight iron in a special manner. In general, the alloying addition may be characterized as having adequate solubility in liquid aluminum but very low or limited solubility in solid aluminum. Thus, when in the condition in which it is used as a conductor, almost the entire addition of iron will be present as intermediate phase particles. In this form, the iron is not as deleterious to electrical conductivity as it would be as a solute in the aluminum solid solution. However, in the usual procedures for processing aluminum alloys, the presence of significant quantities of iron results in the formation of gross particles of the intermediate phase during freezing. Such particles contribute little or are actually deleterious to the mechanical properties of the alloy and often make it difiicult to produce a sound wrought product. It has been found that if the size of the intermediate phase particles formed during freezing can be kept small, i.e., if a cast structure can be obtained comprising an aluminum-rich matrix containing a uniform distribution of fine intermediate phase particles, and this structure is retained during processing, then this alloy of iron with aluminum actually has improved properties. It has been found that the alloys of the invention having iron in excess of 1%, particularly 1.5% Fe and above, have exceptional stability and strength at elevated temperatures.

The findings of this invention include (a) an economically acceptable method of producing and retaining a uniform and sufliciently fine dispersion of intermediate phase particles and (b) alloys containing iron over a range in excess of 1% to about 3%, with the economically acceptable dispersions, possess strength and ductility properties, at normal and elevated temperatures, especially well suited for many electrical conductor applications.

According to the instant invention, a homogeneous melt of substantially pure aluminum and an amount of iron ranging from in excess of 1% to about 3% by weight of iron is prepared. The metal is cast by a method involving quick chilling so that the iron that precipitates during casting forms small evenly distributed intermediate phase particles in the aluminum matrix. The remainder of the iron is present as a solute in a supersaturated solid solution with aluminum. At some stage in the processing subsequent to casting, the metal is heated to an annealing temperature, e.g., from about 500 F. to about 900 F., for a period of time sufficient for most of the iron in supersaturated solution to precipitate as an intermediate phase. This precipitation may develop new particles of the iron-aluminum phase as well as take place on the already formed and uniformly distributed small iron-aluminum particles formed during casting. In this way, the iron will impart strength and ductility to the material without prohibitively impairing its conductivity. Preferably, the melt will contain from 1.5 to 2% iron and most desirably about 2% iron.

The preferred annealing temperature within the above stated range is about 750 F. After the annealing treatment, the rate of cooling to room temperature is not critical. For example, a controlled cooling rate of 100 F./ hr. to 400 F. followed by cooling in air to room temperature or air cooling directly from the annealing temperature have been suitable. The solidified metal may be mechanically worked before heating to the annealing temperature desired. Alternatively, the required heating or portions thereof and working may be accomplished simultaneously. After annealing, the material may or may not be subjected to cold reduction in area as required to produce the desired strength in the final product. For example, when the alloy contains about 2% iron, about 93% cold reduction in area after annealing produces a wire having a tensile strength of about 29,000 p.s.i., a tensile elongation in 10 inches of 4% to 5%, and an electrical conductivity of not less than about 59% LACS. When a wire containing 1.7% iron, prepared by a combination of working and heating, was given a final heat treatment at 750 F., very desirable combinations of strength ductility and electrical conductivity were obtainable. For example, the following ranges of properties were obtained:

Tensile Strength18 k.s.i. to 22 k.s.i.

Yield Strength-12 k.s.i. to 19 k.s.i.

Elongation in inches-17% to 22% and electrical conductivity at least 59% IACS The freezing rate during the casting operation should be sutficient to insure that the iron which precipitates during solidification is present as intermediate phase particles whose greatest dimension is between 0.0002 inch and 0.00004 inch. The actual freezing rate required to meet this condition depends upon the iron content of the alloy. The higher the iron, the faster it must be frozen. The necessary rate of freezing is easily exceeded in properly controlled direct chill casting. For example, when an alloy containing 2% iron is direct chill cast as a one-inch diameter round at a drop rate of 17 inches per minute, the metal is frozen fast enough to limit the size of the cells of the dendritic structure to dimensions of less than 0.00035 inch. With proper casting conditions, bodies of larger cross section, such as 4 inch by 4 inch and 6 inch by 6 inch, can also be cast which have the proper dendritic cell and iron constituent sizes.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows the effect of heating on the conductivity and hardness of cold worked aluminum-2% iron alloy rod. The conductivity as-swaged and the hardnes asswaged and as-cast are also shown thereon.

DETAILED DESCRIPTION As has been discussed, the instant invention relates to an aluminum electrical conductor material having both strength and ductility which are obtained by cold working an aluminum alloy containing a relatively uniform distribution of small particles of the iron-aluminum phase. The processing of the metal is controlled to produce small evenly distributed particles of the iron-aluminum phase initially and then to give enough mobility to the bulk of the dissolved iron in the aluminum to cause it to come out of solution either in the form of new small particles or on the already established particles. All of the intermediate phase particles in the metal are small enough to influence the strength and ductility of the aluminum, but at the same time too large to cause a prohibitive loss in the conductivity of the aluminum matrix.

Example I As an example of an application of this invention, oneinch diameter rounds of an alloy of aluminum containing about 2% iron were cast. The alloy rods were direct chill cast at a drop rate of 17 inches per minute. Without special techniques, conventionally cast material would usually have a slower freezing which yields large, brittle primary particles of the iron-aluminum phase, FeAl in this alloy. Metallographic examination of the internal structure of rapidly frozen rods showed that the iron-aluminum constituent was very fine and was evenly distributed through the matrix of the rod. The maximum dimension of substantially all of the intermediate phase particles present was observed to be between 0.0002 inch and 0.00004 inch. One of these rods was then lightly scalped and cold reduced by swaging to a diameter of 0.375 inch with no intermediate anneal. At this point, the conductivity of the rod was measured and found to be approximately 54% IACS. Segments of the swaged rod were heated at temperatures from 600 F. to 900 F. and, as shown in the drawing, the electrical conductivity increased and the hardness decreased with the temperature of heating. The increase in conductivity is greater than can be accounted for by the softening alone. This indicates that the freezing rate of the cast rod was sufiicient to prevent the diffusion of all of the iron out of the aluminum matrix during casting. In other words, the aluminum matrix was initially supersaturated with iron which as a solute caused a pronounced reduction in electrical conductivity. The heating permitted diffusion and precipitation which reduced the iron level of the matrix. The maximum solid solubility of iron in aluminum is reported to be about 0.05% at the eutectic temperature of 1210 F. It decreases rapidly to only about 0.006% at 932 F. Metallographic examination of the heated samples showed that the size of the intermediate phase particles increased somewhat at the higher annealing temperature.

A %-inch diameter sample rod of this material was annealed for two hours at 700 F. It was subsequently cold drawn to a diameter of 0.100-inch or, in other words, subjected to a 93% cold reduction without any additional in-process anneals. The tensile properties of this wire are listed, along with the typical tensile properties of electrical conductor grade (EC) alloy and the minimum properties of other aluminum base alloys used for conductors, in Table I.

TABLE I Comparison of tensile properties of 2% iron and conductor alloys 0.1-in. diameter wires Aluminum alloy 5005 contains nominally 0.8% Mg and 6201 contains nominally 0.7% Si and 0.8% Mg. The balance in both instances being aluminum and small amounts of impurity elements. H19 and T81 are temper designations in acordance with the Alloy and Temper Designation Systems for Aluminum (USAS H35.ll967). -Hl9 indicates the material has been strain hardened to an extra hard temper. -T81 indicates the material was solution heat treated, cold worked, and then artificially aged.

A comparison of these data shows that the aluminum 2% iron alloy has somewhat better tensile strength than EC alloy with about three times the tensile elongation and only about 2 percentage units less electrical conductivity. The increased ductility is especially important because twisted and/or wrapped connections are often required in conductors. The alloy is not as strong as either 5005 or 6201. However, it has superior ductility and superior electrical conductivity. The 2% iron alloy wire is substantially less costly to manufacture than the high strength 6201-T81 wire.

Example II Additional tests were made to demonstrate the ability of the electrical conductor material of the invention to retain high strength after exposure to high temperatures.

One-inch diameter rounds of an alloy containing 1.7% iron, balance substantially pure aluminum, were cast using the casting procedure set forth in Example I. For comparison purposes, one-inch diameter rounds of an alloy containing 0.75% iron and 0.30% silicon, balance substantially pure aluminum, were also cast according to the same casting procedure. The rounds, after being lightly scalped, were swaged, without an intermediate anneal, to %-inch diameter rods. The rods were then subjected to annealing treatments and their electrical conductivities measured. In the case of the 1.7% iron alloys, some of the rods were annealed at 700 F. for four hours, while others were annealed at 400 F. for four hours. These results are given in Table II.

TABLE II Conductivity of rod, percent Alloy Treatment of rod IACS 0.75% Fe0.30% Si 550 F./4 hours 61.1 1.7% Fe 400 F./4 hours 55.8 1.7% Fe 700 F./4 hours 59. 6

stood that the invention is not limited thereto and that various changes, alterations, and modifications can be made thereto without departing from the spirit and scope as defined in the appended claims.

What is claimed is:

1. The process for producing ductile, high strength aluminum electrical conductor stock which comprises:

(a) preparing a homogeneous melt of substantially pure aluminum and an amount of iron ranging from in excess of 1% to about 3% by weight;

(b) casting the metal by a method involving quick chilling so that the intermediate phases containing iron which precipitates essentially as FeAl during freezing will be present as small particles whose maximum dimension in the aluminum matrix is between 0.0002 inch and 0.00004 inch and the remainder of the iron is present as a solute in a supersaturated solid solution with aluminum;

(c) heating the solidified metal to an annealing temperature from about 500 F. to about 900 F. for a period of time sufiicient for most of the iron in supersaturated solution to diffuse and precipitate either as new particles or on the already formed ironaluminum particles, whereby the iron imparts resistance to softening during heating and strength and ductility to the material without prohibitively reducing its conductivity.

2. The process of claim 1 wherein the melt contains about 2% by weight of iron.

3. The process of claim 1 wherein the melt contains 1.7% by weight of iron.

TABLE TIL-PROPERTIES AT ROOM TEMPERATURE (0.140-INCH DIAMETER WIRE) After Heating at 750 F. for 1 Hour As Drawn After Heating at 750 F. for Hr.

E1ong., Elong., A Elong., Treatment of TS, YS, percent TS, YS, percent Percent TS, YS, percent Alloy %-in. rod ksi ksi in lll-in. ksi" ksi* in 10-in. IACS ksi ksi in l0-in 1.7% Fe 400 11/4 hours.- 36. 4 32. s e. s 21.8 19. 0 17. 6 59.6 21.3 17.6 17. 5 1.7% Fe 700 F./4 hours 29.2 25.6 r7 18.4 12.8 20. 0 50.1 18.0 12.2 22.0 0.75% F90.30% s1 550 F./4 hours 27. 2 25.0 a 4 15. 0 6. 8 28. 6 50. a

From the data given in Table III, it is shown that in 4. The process of claim 1 including the additional step: the case of the alloy containing 1.7% iron after heating at (a) cold working the solidified metal to its final shape. 750 F. for either */2 hour or 1 hour, the yield strength 5. The process of claim 1 wherein the annealing temis well above 10,000 p.s.i. The alloy containing 0.75% perature is about 750 F. iron, 0.30% silicon retained a yield strength of only 6.8 6. The process of claim 1 including the additional step: lc.s.i. In addition, it is known that similar exposures at (a) work-hardening the solidified metal before heating 750 F. of wires of -ECHl9, 5005-H19 and -6201-T81 to the annealing temperature. would result in retained yield strengths of about 4 k.s.i., 6 7. The process of claim 1 wherein the metal is conlc.s.i., and 7 k.s.-i., respectively. A highly desirable, but tinuously cast in the form of a one-inch diameter round heretofore unattainable, characteristic of aluminum magat a drop rate of 17 inches per minute and is subsequently net wire for large transformers would be the ability to heated at the annealing temperature for about two hours. retain a yield strength of 10,000 p.s.i. after an exposure 8. The process of claim 5 wherein heating and workto 750 F. for /2 hour. Not only did the material of this ing are accomplished simultaneously. invention attain this goal but also demonstrated outstand- 9. The process of claim 5 wherein the annealed maing ability to endure even longer exposures at 750 F. terial is cooled in air after heating. with only small additional losses in retained properties. 10. The process of claim 5 wherein the annealed ma- Thus, the material of this invention, after being heated terial is cooled at about 100 F. per hour to 400 F. at 750 F., can be expected to show outstanding stability and then air cooled. during subsequent exposures at temperatures below 750 11. The process of claim 1 wherein the annealed ma- F. Further, it is to be noted that the 1.7% iron alloy, terial is subjected to at least about a 50% cold reduction in addition to the superior properties retained after exin area. posure to high temperature, had a satisfactory electrical 12. A wrought aluminum electrical conductor consisting conductivity, namely, above 59% IACS. Also, the 1.7 essentially of substantially pure aluminum and amount of iron material was determined to have outstandingly exiron ranging from in excess of 1% to about 3% by Weight cellent ductility as evidenced by the high tensile elongaand having metallurgical structure produced by casting intion and the results of bendability tests which were also vOlVing quick chilling wherein the intermediate P e C conducted. taining iron which precipitates during freezing essentially The tensile properties were determined by American as FeAl is present as small particles of between 0.0002 Society for Testing Materials Standard Procedures for iIlCh and 030004 inch in a im m dimension in the Wire Specimens. All conductivity measurements were aluminum matrix followed by an annealing treatment at made at 70 F. with a Kelvin bridge. 500-900 F. wherein substantially all of the iron remaining While there have been shown and described hereinabove in supersaturated solution after freezing is ditfused and possible embodiments of this invention, it is to be underprecipitated either as new particles or on the already formed iron-aluminum particles, said conductor characterized in the wrought condition by strength, ductility and a resistance to softening when heated at elevated temperatures which is superior to aluminum conductor alloys EC, 5005 or 6201.

13. An electrical conductor according to claim 12 wherein the amount of iron ranges from 1.5-2%.

References Cited UNITED STATES PATENTS 8 OTHER REFERENCES Harrington, Trans. of ASM, vol. 41, ,1949, pp. .443-459.

Aluminum, 31 Jahrg. 1955-2, pp. 51-55. 7

Metal Progress, May 1958, pp. 70-76, 176 and 178.

5 Journal of Inst. of Metals, Gwyer et al., vol. 38, No. 2,

1927, pp. 34-47 and 70-75. I

Dix, Proc. for Amer. Soc. for Testing Materials, 28th Annual Meeting, vol. 25, 1925 pp. 120-129.

10 CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 

